Front-end resist stripping historically has been completed with inorganic oxidizing mixtures. Inorganic acids such as sulfuric acid (H.sub.2 SO.sub.4), nitric acid (HNO.sub.3), chromic acid (H.sub.2 CrO.sub.4), phosphoric acid (H.sub.3 PO.sub.4) and hydrogen peroxide (H.sub.2 O.sub.2) have been used since the early days of semiconductor manufacturing to strip resist layers as discussed in L. H. Haplan and B. K. Bergin, Residues from Wet Processing of Positive Resists, J. Electrochem. Soc., 127, 386, 1980. Even today, all of these chemicals can be found in use for resist stripping. Sulfuric/chromic and sulfuric/nitric mixtures are typically used at 100.degree. C. Fuming nitric, also still in use today, is typically used at room temperature. Some of these inorganic oxidizing mixtures can also be used in the back-end of line of wafer processing since many of the concentrated acids are not-corrosive to metals if the water concentration is low.
The use of photoresist chemical baths that contain sulfuric acid and hydrogen peroxide is known as described in U.S. Pat. No. 3,900,337 to Beck, et al. This patent describes the use of a bath containing a relatively low concentration of hydrogen peroxide with respect to the concentration of sulfuric acid. It is stated that the chemical bath should contain a ratio of sulfuric acid:hydrogen peroxide of greater than 15:1, and generally between about 17:1 to about 35:1. It is noted that a combination of two different baths can be used to clean wafers where the first bath contains a higher ratio than the second bath.
Since 1980, Piranha wet baths, composed of sulfuric acid mixed with hydrogen peroxide have gained popularity, and have become the most common method of post-ash resist stripping. Today, the bulk of the resist is usually removed by ashing the photoresist. Therefore, wet photoresist stripping is mainly limited to post-ash resist stripping. Both post-ash resist stripping and bulk photoresist stripping are considered here.
As noted in U.S. Pat. No. 3,900,337, when mixing hydrogen peroxide and sulfuric acid, there is formed "Caro's acid" (H.sub.2 SO.sub.5). Caro's acid is the common name for peroxymonosulfuric acid (i.e. monopersulfuric acid). The chemical structure is shown below: ##STR1##
Caro's acid has an oxidation state of +8 and is the active species in Piranha baths. Concentrated sulfuric acid is an excellent solvent for Caro's acid, whereas the acid decomposes in water. The stripping solution originally developed for Piranha baths is created by a mixture of concentrated H.sub.2 SO.sub.4 with highly concentrated (85-90%) hydrogen peroxide. The reaction that occurs upon mixing proceeds as follows: EQU H.sub.2 O.sub.2 +H.sub.2 SO.sub.4 &lt;.dbd.&gt;HO--(SO.sub.2)--O--OH+H.sub.2 O[1]
Reaction [1] shows that H.sub.2 O is produced in the reaction of H.sub.2 O.sub.2 and H.sub.2 SO.sub.4 and therefore, the presence of water will actually shift the reaction towards the reactants, minimizing the production of Caro's acid. While reaction [1] is not highly exothermic, substantial heat can be developed during the dilution of sulfuric acid with the water that is typically contained in hydrogen peroxide, as well as the water formed by the reaction. Since H.sub.2 SO.sub.5 is not particularly heat-stable, excessive amounts of water in the hydrogen peroxide can lead to thermal decomposition of the H.sub.2 SO.sub.5. Consequently, the production of Caro's acid in Piranha baths is optimized by using hydrogen peroxide in a highly concentrated form (85-90%).
Caro's acid has several major advantages as a photoresist stripper: it can be used at room temperature, and at room temperature and in the absence of water, it is non-corrosive. However, highly concentrated hydrogen peroxide is extremely dangerous; it is potentially detonatable (upon admixture of only small amounts of organics) and can also present a serious fire hazard. For improperly clothed personnel, concentrated H.sub.2 O.sub.2 can cause severe chemical burns.
As a result of safety issues associated with concentrated H.sub.2 O.sub.2, the semiconductor industry has generally adopted the use of "laboratory concentrated" H.sub.2 O.sub.2 (approximately 31% by weight) for wet processing, including Piranha stripping. The use of this dilute H.sub.2 O.sub.2 in a Piranha bath, however, leads to overheating upon mixing. Additionally, the excess water found in 31% by weight H.sub.2 O.sub.2 shifts the equilibrium in reaction [1] away from the production of H.sub.2 SO.sub.5. Therefore, the use of 31% by weight H.sub.2 O.sub.2 in a Piranha bath does not produce Caro's acid to any significant concentration. As a result, the mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 has to be heated to very high temperatures in order to be effective in resist stripping. Typically, temperatures as high as 120.degree. C. are used. Such high temperatures help to drive the water off. Solution dilutions are generally about 4:1, 4 parts H.sub.2 SO.sub.4 to 1 part H.sub.2 O.sub.2 (H.sub.2 O.sub.2 being at 31% strength). As a result of the instability of this solution, hydrogen peroxide has to be added continuously, thereby adding 69% water with every spiking, thus making the solution even more instable. Consequently, frequent exchange of the Piranha baths are required.
Recently, ozone has been introduced as an alternative to hydrogen peroxide in Piranha baths. From a consumables point of view, replacing H.sub.2 O.sub.2 by O.sub.3 is very attractive as H.sub.2 O.sub.2 is one of the most expensive chemicals, whereas O.sub.3 can be generated in-situ from O.sub.2 gas or even from air.